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Vaporstat Settings

I have a 20's, 2-pipe, vapor, gravity return system served by a recent Weil-McLain EGH-95 boiler (320,000 gross BTU rating) and equiped with 2 in-line Pressuretrols.  They are both set to 0.5psi cut-in (setting bottomed out) and 1.5 psi-cut-out and are operating fine but hard to tell as pressure never registers on 30psi gauge. 

I'm adding a low pressure gauge to better monitor pressure and have seen a lot of recommendations on the boards and in the books for installing a Vaporstat but no guidance on how to manage these controls if different than the Pressuretrol. 

The system should have no more than 4 oz of pressure loss line runs (assuming 2 oz per 100' for 2-pipe systems and 100% adder for fittings/valves).  That may be on high end suggesting that I may be able to lower cut-in to less than 8oz. 

A couple of questions to ensure I know what to do when I replace one of the Pressuretrol's with a Vaporstat:

-I assume I replace one Pressuretrol and leave the other in place as a backup, correct?

-Should I raise the cut-in and cut-out pressures on the remaining Pressuretrol to take them out of range for the Vaporstat (i.e., 1 psi cut-in and 3 psi cut-out)?

-Since their wired in-line, I also assume it does not matter which Pressuretrol I replace, correct?

-Should I set the Vaporstat at the same cut-in / cut-out pressures or try to lower further?  I'm not clear if that is part of the advantage of the Vaporstat or just that I should get better accuracy at these low pressures.

Thanks for the input.


  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Lower Setting is Often Desirable

    I'm assuming this is a steam system, not a "vapor" system.

    All your conclusions are correct.

    The Vaporstat is usually set lower than a Pressuretrol, say 0.5psi to 1.5psi .

    Most systems will work a little better, but some systems could have trouble with lower settings. Some of these problems are described in Dan's books.

    Here's my take on why the Vaporstat is the better control:

    The lower pressure of the Vaporstat allows a lower average temperature of the boiler, which is more efficient, and still creates enough steam to reach the whole building.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • cn30
    cn30 Member Posts: 34
    Vaporstat follow-up

    If a system runs on low pressure due to correct boiler sizing, piping, and good venting, is a vaporstat needed? It won't lower the pressure itself, all it seems it could do is let you set a lower cut-out pressure, which would cause you to cycle more often on pressure. Is that ever a plus, as long as your pressure is low to begin with?
  • SteamySwede
    SteamySwede Member Posts: 20
    Lower Cut-in?

    Kevin - I'm no expert but per Dan's books, what I have is a vapor system, but not a vapor/vacuum system.  It runs on steam at low, but positive, pressure.  Mine is 2-pipe with traps on all the radiators.

    CN30 - again, I'm no expert, but my takeaway is that there are two reasons to use the Vaporstat:

    1) every type of gauge or control I've seen over the years tends to be less accurate at the extremes of its range.  My Pressuretrols bottom out at 0.5psi cut-in.  I'm not sure how accurate that is in reality given that we're just bottoming out the control.  I would expect the Vaporstat that has this value more in the narrower "operating" pressure range will be more accurate even at the same setting. 

    2) the Vaporstat would also permit an even lower cut-in pressure than the Pressuretrol.  Per Dan's books, the desired cut-in is 2x the pressure loss before reaching the furthest radiator.  After rereading this section of a book, I'm estimating my pressure drop at 2 oz.  so a 4 oz cut-in is theoretically possibly (so maybe a 20 oz cut-out). That's 1/2 of the cut-in pressure where I am currently and should be more efficient in terms of boiler firing (less pressure to build and faster moving steam).  I don't think that it leads to more short cycling if you continue to use a 1 psi differential before cut-out.   I think the lower the pressure you can operate at and achieve the desired heating (and return condensate at), the more efficient the system.

    Again, I'm a novice so if I'm off the mark I appreciate being corrected.
  • cn30
    cn30 Member Posts: 34

    I'm new to this also, so I'm just going on logic (or what seems to me to be logic). I would think that the ideal is to run at a low pressure so that your system cuts in and out based solely on the thermostat. In that case, your Pressuretrol could be set at 8 psi with a cut-out at 9, and your system would just ignore it and run on the thermostat. Putting aside safety issues (which you can't, of course), I would think you could also theoretically run it without a Pressuretrol (or vaporstat) at all.  Again, your system would go on when the thermostat called for heat, and go off when it was satisfied. To me they appear to be safety devices, and efficiency devices in case your system cycles on pressure.

    Anyone else, thoughts?
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Slow Down that logic


    "To me they appear to be safety devices, and efficiency devices in case your system cycles on pressure."

    A steam boiler will always cycle on pressure, except at design conditions (-5F for cold cities) since the flame doesn't modulate. A 500,000 btu boiler will always have a 500,000 btu flame when it is firing. So if the pressuretrol is set to 10psi, the boiling point is about 235F, and causing the boiler to operate at a much reduced efficiency.

    Here's what I wrote over at the other vaporstat thread:

    In the general field of heating, short cycling usually causes a drop in efficiency.

    In the case of steam, it doesn't hurt efficiency. Here's why:

    The most efficient steam boiler would fire right at the boiling point, never below, and never above. The boiling point is the lowest possible temperature for the boiler to operate at. If it is below the boiling point, no steam is being delivered to the load. If it isn't delivering heat to the load, it's efficiency is almost zero.

    If the boiler keeps firing and raises the pressure more than necessary, it is operating above the temperature it needs to be at to satisfy the load. If it's hotter than it needs to be, it is losing more heat than it needs to, through the jacket and the flue, and that drags down efficiency.

    Last year I replaced my Pressuretrol with a Vaporstat, and this tighter control saved about $400 worth of gas on my old 1mmbtu Peerless. But it definitely cycled faster.

    One downside: short cycling will wear out the control faster. But the savings more than make up for that.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • Kevin_in_Denver_2
    Kevin_in_Denver_2 Member Posts: 588
    Right, 1psi differential should cycle the same


    Your thinking is correct. I just used the 0.5psi to 1.5psi suggestion from Dan's books. The increase in efficiency was great, so I haven't tried to go to a lower pressure range.
    Superinsulated Passive solar house, Buderus in floor backup heat by Mark Eatherton, 3KW grid-tied PV system, various solar thermal experiments
  • BobC
    BobC Member Posts: 5,478
    Could be lower

    My system is a small single pipe setup and I've been using a 12 to 4 oz range for a few years now on both the old v75 and my new Smith G8 and the system is running fine.

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
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